Polymer nanocomposite foams

ABSTRACT

Nano-sized particles such as nano-clays can be mixed with polymers through either melt compounding or in-situ polymerization. By modifying the particle surface with various surfactants and controlling processing conditions, we are able to achieve either intercalated (partial dispersion) or exfoliated (full dispersion) nano-clay distribution in polymers with the clay content up to 35% by weight. When a blowing agent is injected into the nanocomposite in an extruder (a continuous mixer) or a batch mixer, polymeric foam can be produced. Supercritical carbon dioxide, an environmentally friendly, low-cost, non-flammable, chemically benign gas is used as the blowing agent. This process forms a microcellular foam with very high cell density (&gt;10 9  cells/cc) and small cell size (&lt;5 microns) can be achieved by controlling the CO 2  content, melt and die temperature, and pressure drop rate.

[0001] The present invention hereby incorporates by reference,provisional application No. ______,entitled “Clay NanocompositesPrepared by In-situ Polymerization”, filed on Apr. 29, 2002.

[0002] The present invention was made with Government support underGrant No. ______ awarded by the ______. The United States Government mayhave certain rights to this invention under 35 U.S.C. §200 et seq.

TECHNICAL FIELD OF THE INVENTION

[0003] The present invention is in the field of polymer foams.Specifically, the present invention relates to polymer nanocompositefoams.

BACKGROUND OF THE INVENTION

[0004] Foamed polymers are found in applications ranging from packaging,insulation, cushions, adsorbents, to scaffolds for tissue engineering.The basic principle of foaming is to mix a blowing agent (typically agas) into a polymer melt and induce a thermodynamic instability througha temperature or pressure change to nucleate bubbles of the blowingagent.

[0005] In this invention, supercritical CO₂ (the critical temperatureT_(c): 31° C. and the critical pressure P_(c): 73.8 bar or 1074 psi), apotential replacement of the traditional foaming agents (hydrocarbon orchlorofluorocarbon), was applied, The liquid-like solubility andgas-like diffusivity make it possible to dissolve sufficient CO₂ in apolymer quickly. CO₂ is low-cost, non-flammable, chemically benign, andenvironmentally friendly.

[0006] Recently, microcellular foams, characterized by cell sizessmaller than 10 μm and cell density larger than 10⁹ cells/cm³, havedrawn a great deal of attention and interest. It has been shown that bykeeping the cell (or bubble) size uniformly less than 10 microns indiameter, one can greatly reduce material usage without compromisingmechanical properties because the bubbles are smaller than thepreexisting flaws in a polymer matrix.

[0007] The field of polymer/clay nanocomposites has grown rapidly in thepast decade. In this work, nano-sized particles, nanoclays, are appliedto modify the cellular foams in both batch and continuous extrusionfoaming process. The results show that with the addition of a very smallamount of nanoclay into the polymer matrix, the nanocomposites exhibitsubstantial increase in many physical properties, including mechanicalstrength (tensile modulus and strength, flexural modulus and strength),thermal stability, flame retardance, and barrier resistance. Smectiteclays, such as montmorillonite (MMT), are of particular interest becausethey have a high aspect ratio (lateral dimension ˜200-500 nm, thickness<1 nm) and a high surface area. However, clay is hydrophilic in natureand incompatible with most polymers. To increase the compatibility andmiscibility of clay in polymer, the clay surface is modified by anorganic surfactant, typically ammonium cations with long alkyl chains.

[0008] Two idealized polymer/clay structures are possible: intercalatedand exfoliated. Exfoliation involves extensive polymer penetration todisrupt the clay crystallite (tactoids), and the individualnanometer-thick silicate platelets are dispersed in the polymer matrix.If there is only limited polymer chain insertion in the interlayerregion, and the interlayer spacing only expands to a certain extentwithout losing layer registry, then an intercalated nanocomposites isthen formed.

[0009] Polymer foam is another area subject to intensive research. It iswidely used for insulation, packaging, and structural applications, toname a few. Microcelluar foam, which is characterized by cell size inthe range of 0.1˜10 μm, cell density in the range of 10⁹ to 10¹⁵cells/cc, provides improved mechanical properties as well as increasedthermal stability and lower thermal conductivity.

[0010] Cell nucleation and growth are two important factors controllingcell morphology. Particles can serve as a nucleation agent to improveheterogeneous nucleation. Some inorganic nucleation agents, such astalc, silicon oxide, kaoline, etc., are widely used. A fine dispersionof these nucleation agents can promote formation of nucleation centerfor the gaseous phase. Although a detailed explanation of theheterogeneous nucleation mechanism is still not available, the size,shape, and distribution, and surface treatment of particles have greatinfluences on the nucleation efficiency. In this work, we developed anew polymer nanocomposite foam preparation technology to create polymerfoams with controlled cell structure. In addition, clay may furtherimprove the foam properties, e.g., mechanical and barrier properties, aswell as fire resistance.

SUMMARY OF THE INVENTION

[0011] The present invention includes polymeric nanocomposite foams anda method for forming polymeric nanocomposite foams.

[0012] A method for forming a polymeric nanocomposite foam of thepresent invention comprises the steps of providing a mixture comprising:a polymer, an organophilic clay, and a blowing agent; and processingsaid mixture so as to cause formation of cells, thereby forming apolymeric nanocomposite foam.

[0013] Although any appropriate amount of blowing agent may be used, itis preferred that the mixture comprises at least 1% by weight of theblowing agent. It is more preferred that the mixture comprise at least4% by weight of the blowing agent. It is most preferred that the mixturecomprises at least 7% by weight of said blowing agent.

[0014] Although any desired amount of organophilic clay may be used, itis preferred that the mixture contain at least 0.5% by weight of theorganophilic clay. It is more preferred that the mixture comprise atleast 5% by weight of the organophilic clay. It is further preferredthat the mixture comprise at least 10% by weight of the organophilicclay. It is most preferred that the mixture comprises at least 20% byweight of the organophilic clay.

[0015] While any appropriate polymer may be used in forming thepolymeric nanocomposite foam, it is preferred that the polymer isselected from the group consisting of polystyrene, poly(methylmethacrylate), polypropylene, nylon, polyurethane, elastomers, andmixtures thereof.

[0016] It is preferred that the organophilic clay is dispersedthroughout the polymer such that a x-ray diffraction pattern producedfrom the mixture is substantially devoid of an intercalation peak forproducing exfoliated polymeric nanocomposite foams. It is preferred thatorganophilic clay is dispersed throughout the polymer such that a x-raydiffraction pattern produced from the mixture contains an intercalationpeak for producing intercalated polymeric nanocomposite foams.

[0017] It is preferred that the organophilic clay comprises: a smectiteclay; and a compound having the formula:

[0018] wherein R1 is (CH)_(n) wherein n ranges from 6 to 20; R2 is achemical structure having a terminal reactive double bond; R3 is analkyl group; and R4 is an alkyl group.

[0019] It is most preferred that the compound have n=15, R3 as CH₃, R4as CH₃, and R2 as:

[0020] While any appropriate clay may be used, it is preferred to usesmectite clay. It is more preferred that the smectite clay is selectedfrom the group consisting of montmorillonite, hectorite, saponite,laponite, florohectorite, and beidellite.

[0021] The blowing gas may be any traditional blowing gas used inindustry (for example: freon, nitrogen or air). However, it is preferredthat the blowing agent is a supercritical fluid. It is most preferredthat the blowing agent is supercritical carbon dioxide.

[0022] Cell size can vary widely depending upon operating conditions,however, it is preferred that the polymeric nanocomposite foam has anaverage cell size less than about 20 microns. It is additionallypreferred that the polymeric nanocomposite foam has an average cell sizegreater than about 15 microns.

[0023] Cell density can vary widely depending on operating conditions,however, it is preferred that the polymeric nanocomposite foam has anaverage cell density greater than about 1×10⁶ cells/cm³. It is morepreferred that the polymeric nanocomposite foam have an average celldensity greater than about 1×10⁹ cells/cm³.

[0024] The polymer nanocomposite foam may be closed cell foam or opencell foam.

[0025] A polymeric nanocomposite foam of the present invention comprisesa polymeric portion; an organophilic clay, the organophilic clay isdispersed throughout the polymeric portion; and a plurality of cellsdispersed throughout the polymeric portion.

[0026] While any appropriate polymer may be used in the polymericnanocomposite foam, it is preferred that the polymeric portion comprisesa polymer selected from the group consisting of polystyrene, poly(methylmethacrylate), polypropylene, nylon, polyurethane, elastomers, andmixtures thereof.

[0027] It is preferred that the organophilic clay is dispersedthroughout the polymer such that a x-ray diffraction pattern producedfrom the mixture is substantially devoid of an intercalation peak forexfoliated polymeric nanocomposite foams. It is preferred thatorganophilic clay is dispersed throughout the polymer such that a x-raydiffraction pattern produced from the mixture contains an intercalationpeak for intercalated polymeric nanocomposite foams.

[0028] While any organophilic clay may be used, it is preferred that theorganophilic clay portion comprises: a smectite clay; and a compoundhaving the formula:

[0029] wherein R1 is (CH)_(n) wherein n ranges from 6 to 20; R2 is achemical structure having a terminal reactive double bond; R3 is analkyl group; and R4 is an alkyl group.

[0030] It is most preferred that the compound have n=15, R3 as CH₃, R4as CH₃, and R2 as:

[0031] While any appropriate clay may be used, it is preferred to usesmectite clay. It is more preferred that the smectite clay is selectedfrom the group consisting of montmorillonite, hectorite, saponite,laponite, florohectorite, and beidellite.

[0032] Cell size can vary widely depending upon operating conditions,however, it is preferred that the polymeric nanocomposite foam has anaverage cell size less than about 20 microns. It is additionallypreferred that the polymeric nanocomposite foam has an average cell sizegreater than about 15 microns.

[0033] Cell density can vary widely depending on operating conditions,however, it is preferred that the polymeric nanocomposite foam has anaverage cell density greater than about 1×10⁶ cells/cm³. It is morepreferred that the polymeric nanocomposite foam has an average celldensity greater than about 1×10⁹ cells/cm³.

[0034] The polymer nanocomposite foam may be closed cell foam or opencell foam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 shows the chemical structure of the surfactant2-methacryloyloxyethylhexadecyldimethyl ammonium bromide (MHAB).

[0036]FIG. 2 provides XRD patterns for various PS/clay nanocomposites.

[0037]FIG. 3 is a TEM micrograph of intercalated PS/20A nanocompositedemonstrating the large clay aggregates that are still present in thematrix.

[0038]FIG. 4 is a TEM micrograph of exfoliated PS/MHABS nanocompositeshowing how the tactoids have been completely delaminated and uniformlydispersed.

[0039]FIG. 5 provides XRD patterns for various PMMA/clay nanocomposites.

[0040]FIG. 6 is a SEM micrograph of polystyrene foam produced by a batchfoaming procedure.

[0041]FIG. 7 is a SEM micrograph of PS/5% 20A foam produced by a batchfoaming procedure.

[0042]FIG. 8 is a SEM micrograph of PS/5% MHABS foam produced by a batchfoaming procedure.

[0043]FIG. 9 compares cell size and cell density for PS and PS/claynanocomposite foams.

[0044]FIG. 10 is a SEM micrograph of PMMA foam produced by a batchfoaming procedure.

[0045]FIG. 11 is a SEM micrograph of PMMA/5% 20A foam produced by abatch foaming procedure.

[0046]FIG. 12 is a SEM micrograph of PMMA/5% MHABS foam produced by abatch foaming procedure.

[0047]FIG. 13 compares cell size and cell density for PMMA and PMMA/claynanocomposite foams.

[0048]FIG. 14 is a SEM micrograph of PS/talc filler foam produced byextrusion.

[0049]FIG. 15 is a SEM micrograph of PS/20A foam produced by extrusion.

[0050]FIG. 16 is a SEM micrograph of PS/MHABS foam produced byextrusion.

[0051]FIG. 17 is a SEM micrograph of PS foam by a batch foamingprocedure.

[0052]FIG. 18 is a SEM micrograph of PS/1% MHABS foam by a batch foamingprocedure.

[0053]FIG. 19 is a SEM micrograph of PS/10% MHABS foam by a batchfoaming procedure.

[0054]FIG. 20 shows the effect of clay concentration on cell size anddensity based upon the concentration of MHABS by a batch foamingprocedure.

[0055]FIG. 21 is a SEM micrograph of pure PS foam.

[0056]FIG. 22 is a SEM micrograph of PS/2.5-wt % 20A foam.

[0057]FIG. 23 is a SEM micrograph of PS/5-wt % 20A foam.

[0058]FIG. 24 is a SEM micrograph of PS/7.5-wt % 20A foam.

[0059]FIG. 25 shows the relationship between pressure drop rate and cellsize for three different materials.

[0060]FIG. 26 shows the relationship between pressure drop rate and celldensity for three different materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0061] In accordance with the foregoing summary, the following presentsa detailed description of the preferred embodiments of the inventionthat is currently considered to be the best mode.

[0062] Materials

[0063] Styrene (St), Methyl Methalcrylate (MMA) and initiator2,2′-azobisisobutyronitrile (AIBN), were purchased from Aldrich. Apolystyrene resin (Fina) was used to prepare nanocomposites byextrusion. Two types of organically modified montmorillonite clays wereused in this study. Cloisite 20A (20A) was donated by Southern Clay. Theinterlayer cation is dimethyl dehydrogenated tallowalkyl ammonium onium.Na⁺-montmorillonite (CEC=90 meq/100 g) was also from Southern Clay. Areactive cationic surfactant 2-methacryloyloxyethylhexadecylditnethylammonium bromide (MHAB) was synthesized and ion-exchanged withNa⁺-montmorillonite to prepare the organophilic clay, according to theprocedure published elsewhere. The chemical structure of the surfactantMHAB is shown in FIG. 1. The modified clay is denoted as MHABS. Praxairprovided the foaming agent, a bone-dry grade of carbon dioxide.

[0064] Preparation of Polymer/Clay Nanocomposites

[0065] Both twin-screw extrusion and in-situ polymerization was used toprepare PS/clay and PMMA/clay nanocomposites. In-situ polymerization wascarried out under isothermal conditions at 60° C. for styrene and 50° C.for MMA. The monomer, clay and a certain amount of AIBN were mixedtogether using a high shear mixer. The mixture was then reacted for 20hrs, then the temperature was raised to 105° C. for another 30 min. 5%exfoliated nanocomposites prepared by in-situ polymerization were usedfor extrusion foaming. 20% exfoliated PS nanocomposite masterbatch wasblended with polystyrene (PS) to prepare exfoliated nanocomposites,using a DACA microcompounder for batch foaming. Intercalatednanocomposites were prepared using a Leistritz ZSE-27 intermesh twinscrew extruder (L/D=40, d=27 mm) operated in the co-rotating mode. Thescrew speed was 200 rpm.

[0066] Foaming of Polymer/Clay Nanocomposites

[0067] Batch foaming was performed at 120° C. CO₂ was delivered via asyringe pump. The system was allowed to equilibrate for 24 hrs for CO₂to reach saturation in the polymer matrix. The pressure was then rapidlyreleased and the foamed cells were fixed by cooling with water. Thesaturation pressure was 2000 psi and the pressure was released in 2-3seconds for cell nucleation.

[0068] The microcellular foaming extrusion was performed on a two-stagesingle-screw extruder (HAAKE Rheomex 252P). A static mixer (Omega,FMX8441S) was attached to the end of the extruder to provide extramixing capacity. A capillary die with a 0.5 mm diameter and 10 mm lengthnozzle was custom made to generate a high and rapid pressure drop. CO₂was delivered from a syringe pump (ISCO 260D) with a cooling jacket. TheCO₂ pressure and volumetric flow rate can be read precisely from thepump controller.

[0069] CO₂ is compressed to a certain pressure in the syringe pump at40° C. reaching a supercritical state. 4-wt % of CO₂ was injected intothe extruder barrel by carefully controlling the pressure and volumetricflow rate of CO₂. Upon injection into the barrel, it is mixed with thepolystyrene melt by screw rotation. A single-phase solution is formedwhen the mixture flows through the static mixer. Nucleation occurs inthe die because of the solubility reduction due to the quick and largepressure drop realized by the narrow capillary nozzle. The foamedextrudate flows freely out to the air and vitrifies.

[0070] Analytical Methods

[0071] The X-ray diffraction (XRD) patterns of prepared polymer/claynanocomposites were recorded on a Scintag XDS-2000 X-ray diffractometerwith Cu Kα radiation and operated at 35 kV and 10 mA. Transmissionelectron microscopy (TEM) image was obtained from a Phillip CM12 usingan accelerating voltage of 80 kV. The nanocomposite samples weremicrotomed at room temperature with a diamond knife and mounted on a 200mesh copper grid. A Phillip XL30 scanning electron microscope was alsoused to observe the cell morphology.

[0072] Results and Discussion: Structure of Nanocomposites

[0073] Montmorillonite clay particles contain thousands of individuallayers with a thickness dimension ˜1 nm and lateral dimension ˜1 μm. Thepolymer chain penetration and interlayer expansion depend on thecompatibility of the polymer matrix and the clay surface. Intercalatednanocomposites usually form when there is only limited insertion of apolymer chain into the interlayer region. This results in the interlayerexpansion and can be detected by x-ray diffraction (XRD). FIG. 2 showsthe XRD of PS/clay nanocomposites. For PS/5% 20A, the shift of thediffraction peak to a lower angle region clearly verifies the polymerchain intercalation. The (d001) basal spacing increased from 2.3 nm to3.4 nm. The TEM micrograph of FIG. 3 demonstrates that large clayaggregates are still present in the matrix. Face-to-face layer stackingand low angle intergrowth of tactoids are still observable. On the otherhand, by using the reactive surfactant MHAB, the copolymerization ofMHAB and styrene monomer helped layer separation and exfoliatednanocomposite was synthesized with a clay concentration up to 20%. TheXRD of PS/5% MHABS and PS/20% MHABS show featureless pattern (FIG. 2). ATEM micrograph of PS/20% MHABS is shown in FIG. 4. The tactoids havebeen completely delaminated and uniformly dispersed in the matrix. Mostclay layers are present as single layers, while stacks of a few layersare also observable in some region. This nanocomposite was then blendedwith PS to make nanocomposites for the foam experiments. FIG. 5 showsthe XRD of PMMA/clay nanocomposites. For PMMA/5% 20A, the shift of thediffraction peak to a lower angle region clearly verifies the polymerchain intercalation. The (d001) basal spacing increased from 2.3 nm to3.6 nm. Again, the diffraction peak disappears for the exfoliatedPMMA/5% MHABS nanocomposite.

[0074] Effect of Clay Dispersion

[0075] Batch foaming experiments were conducted to compare the effect ofdifferent clay dispersions on the foam cell morphology, as shown inFIGS. 6-8. The clay concentration is 5-wt %. With the addition of clay,the cell size decreases and the cell density increases. Image analysiswas used to obtain the average cell size and cell density, and theresult is shown in FIG. 9. In the presence of 5% 20A, the cell sizedecreases from 20 μm to 15 μm, and the cell density increases from8.2×10⁷ to 1.3×10⁸ The exfoliated nanocomposite foam has an average cellsize of 11 μm and cell density of around 4.2×10⁸. The clay may serve asa heterogeneous nucleation agent allowing more sites to nucleate andgrow. This leads to an increase in cell density. While more cells startto grow at the same time, there is less opportunity for the individualcells to grow bigger, leading to a smaller cell size. In intercalatednanocomposites, most clay exists as stacks of layers or tactoids,serving as nucleation sites. On the other hand, in exfoliatednanocomposites, clay is present mostly as individual layers and usuallythe distance between the layers is greater than the effective radius ofgyration of a polymer chain. Unlike in intercalated nanocomposites wherepolymer chain penetration is limited and the major contact area is theouter surface of the tactoids, in exfoliated nanocomposites theindividual layer is in direct contact with the matrix, providing muchlarger interfacial area for CO₂ adsorption and cell nucleation. In otherwords, once exfoliated, the effective particle concentration is higherand the number of nucleation sites increases. As a result, theexfoliated nanocomposite foam shows the highest cell density and thesmallest cell size. Another factor that may affect the cell size anddensity is the rheological properties of the nanocomposites. Further, weobtained extremely small cell size (<1.6 μm) and large cell density(>10¹¹ cells/cm³) when PMMA and its nanocomposites were foamed even in abatch process. The morphology was shown in FIGS. 10-12. The major reasonis believed to be the higher solubility of CO₂ in PMMA. The cell sizeand cell density were compared in FIG. 13.

[0076] Both intercalated and exfoliated PS/clay nanocomposites werefoamed in a single screw extruder. For comparison, PS/talc foams werealso prepared in the same extruder. The cell morphology is shown inFIGS. 14-16. Cells in PS/talc are much larger and the cell density ismuch lower than those in PS/20A at the same particle concentration. Onceexfoliated, the nanocomposite foam has the smallest cell size and thehighest cell density. Exfoliated nanocomposite shows perfectmicrocellular foam structure in which cells are round in shape, closed,and well separated from each other. Very few cell coalescence wasobserved. The calculated average cell size and cell density are 4.9microns and 1.5×10⁹ cells/cm³ respectively. In addition, the exfoliatedcomposite extrudate exhibits a very smooth and shining surface thatcomes from the orderly alignment of the single clay layers, the smallcell size, and the few flaws in the polymer matrix.

[0077] Effect of Clay Concentration

[0078] A series of exfoliated PS/MHABS nanocomposites (1%, 5%, and 10%)were foamed (T=120° C., P=2000 psi) to study the effect of clayconcentration on cell morphology. The SEM micrographs are shown in FIGS.17-19. And the cell size and density are shown in FIG. 20. Adding 1%MHABS greatly reduces the cell size and increases cell density. The cellsize of this nanocomposite foam is comparable to that of 5% 20Ananocomposite foam, while the cell density is higher. This supports ourhypothesis that the individual layers are capable of serving asnucleation sites. Even though the apparent concentration of MHABS islower than 20A, there may be more interfacial area between the clay andthe matrix due to exfoliation. Adding 5% MHABS can further reduce cellsize and increase cell density. However, further increasing the clayconcentration to 10% seems only to increase cell density, while cellsize remains almost unchanged. During foaming, both nucleation andgrowth will affect the cell size and density. And the cell growthdepends strongly on the polymer rheological properties, which areaffected greatly by the presence of clay. Both shear and elongationalviscosity increase when clay is added. The individual layers as well asthe tactoids can form a percolated structure at the mesoscopic level,impeding the motion of the polymer chain and thus increasing the shearviscosity. The exfoliated nanocomposites show a higher viscosity thanthe intercalated nanocomposites. The increase of the viscosity hindersthe cell growth, resulting in a smaller cell size. The reason for thenearly the same cell size for the 5% and 10% nanocomposite foams isunclear. A possible explanation is as follows. When there are more clayplatelets and more cell nucleation, It is more probable for cells tomeet each other and form larger cells. This will lead to an increase incell size.

[0079] We have showed that the addition of clay can help reduce cellsize and increase cell density. However, these nanocomposite foams arestill are in the microcellular foam range. During batch foaming, thepressure drop rate is not high enough, and therefore there is sufficienttime for cells to grow. In the continuous extrusion foaming, theoperating conditions can be controlled to generate a high enoughpressure drop. In fact, microcellular nanocomposite foams were preparedin our lab.

[0080] To investigate the effect of nanoclay on microcellular foamingextrusion, nanocomposites with different 20A concentration (0-10 wt %)were foamed in the single screw extruder under similar operationconditions. The cell size decreases dramatically after a small amount ofnanoclay (˜2.5 wt %) is blended in and then it levels off at high clayconcentration. However, the cell density increases nearly linearly.Exfoliated PS/MHABS nanocomposites with different compositions (0-20 wt.% MHABS) were also foamed. Similar trends in cell size and cell densitywere observed and more small cells were obtained compared withintercalated PS/20A nanocomposites.

[0081] Comparing the SEM images shown in FIGS. 21-24 of samplescontaining 0, 2.5, 5 and 7.5 wt. % of 20A, it is found that the cellstend to coalesce together when more 20A is blended in PS. This may beused to produce open cell foams that are important for adsorption,filtration, and scaffolds of tissue engineering.

[0082] Effect of Operating Conditions

[0083] PS and PS/20A intercalated nanocomposites were also foamed atdifferent pressure drops by changing the screw rotation speed or themass flow rate of the polymer/CO₂ mixture.

[0084] The results are summarized in FIGS. 25 and 26 that exhibit howcell size and cell density change with increasing pressure drop rate. Aninteresting phenomenon is that the decrease of the cell size becomesslower at high pressure drop rates, while the cell density increaseslinearly. Comparing with pure PS, nanocomposites make the microcellularfoaming process easier, where the cell size of nanocomposites can beeasily smaller than 10 μm and cell density larger than 10⁹ cells/cm³after the pressure drop rate is greater than 10⁹ Pa/sec. The exfoliatednanocomposite provides the smallest cells and largest cell density atthe lowest screw rotation speed (or the lowest pressure drop rate, 5×10⁸Pa/s).

[0085] Besides pressure, the influence of CO₂ concentration (0-8 wt %)and foaming die temperature (120-240° C.) was also explored. Below theCO₂ solubility limit, cell size decreases and cell density increaseswith the increase of CO₂ concentration. A high CO₂ concentration isfavorable for producing open cell foams. Die temperature affects bothcell size and cell structure (open or closed).

[0086] Comparing to conventional micron sized filler particles used asnucleation agents in the foaming process, the extremely fine dimensionsand large surface area of nanoparticles and the intimate contact betweenparticles and polymer matrix may greatly alter the cell nucleation andgrowth. It can absorb more CO₂ on its surface. The addition of nanoclayalso increases the viscosity of the polymer matrix. This may increasethe pressure drop rate in the die. The nanoclay can increase the celldensity and change the cell structure (open or closed). This becomesmore prominent when a polymer having low foaming ability withsupercritical CO₂ needs to be foamed. Furthermore, the nanoclay mayimprove the barrier properties (low diffusion coefficient for both massand heat), insulation properties (low heat conductivity), mechanicalproperties, and heat resistance, offering new opportunities in variousapplications.

[0087] Polystyrene/clay and PMMA/clay nanocomposites were prepared andused to make nanocomposite foams. It was found that the cell size isgreatly reduced, and the cell density is increased, by adding a smallamount of clay. The clay dispersion also has a great influence on thecell morphology. The exfoliated nanocomposite foam provides the highestdensity and lowest cell size. For exfoliated nanocomposite foams, ahigher clay concentration seems mainly to improve cell density. Addingclay not only provides sites for nucleation, but also changes therheological properties of the polymer matrix, which is also important infoaming process.

[0088] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiment(s), but on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims, which are incorporated hereinby reference.

[0089] References:

[0090] The following references are hereby incorporated by reference:

[0091] 1. M. Alexandre, P. 1. M. Alexandre, P. Dubois, Mater. Sci. Eng.,28, 1 (2000).

[0092] 2. D. Klempner and K. C. Frisch, eds. Handbook of Polymeric Foamsand Foam Technology; Oxford University Press: Munich; Vienna; New York(1991).

[0093] 3. C. Zeng and L. J. Lee, Macromolecules, 34(12), 4098 (2001).

[0094] 4. X. Han, C. Zeng, L. J. Lee, K. W. Koelling and D. L. Tomasko,Annu. Tech. Conf.-Soc. Plast. Eng., (2002).

[0095] 5. R. A. Vaia, E. Giannelis, Marcomolecules, 30,7990 (1997)

[0096] 6. R. A. Vaia, E. Giannelis, Marcomolecules, 30, 8000 (1997)

[0097] 7. R. Krishnamoorti, J. Ren and A. S. Silva, J. Chem. Phys.,114(11), 4968 (2001).

[0098] 8. M. Okamoto, P. H. Nam, P. Maiti, T. Kotaka, N. Hasegawa and A.Usuki, Proceeding of the First World Conference on Nanocomposites,Chicago, Ill. (2001).

[0099] 9. P. H. Nam, P. Maiti, M. Okamoto, T. Kotaka, T. Nakayama, M.Takada, M. Ohshima and A. Usuki, Proceeding of the First WorldConference on Nanocomposites, Chicago, Ill. (2001).

[0100] 10. A. I. Cooper, J. Mater. Chem., 10 (2), 207 (2000).

[0101] 11. C. B. Park, N. P. Suh, and D. F. Baldwin, Method forProviding Continuous Processing of Microcellular and SupermicrocellularFoamed Materials, U.S. Pat. No. 5,866,053 (1999).

[0102] 12. X. Han, K. W. Koelling, D. L. Tomasko, and L. J. Lee, Annu.Tech. Conf.-Soc. Plast. Eng., 58^(th) (Vol. 2), 2 1857 (2000).

[0103] 13. X. Han, K. W. Koelling, D. L. Tomasko, and L. J. Lee, Annu.Tech. Conf.-Soc. Plast. Eng., 2 1741 (2001).

[0104] 14. C. Zeng, X. Han, L. J. Lee, K. W. Koelling, and D. L.Tomasko, Structure of Nanocomposite Foams, unpublished.

[0105] 15. L. J. Lee, K. W. Koelling, D. L. Tomasko, X. Han, and C.Zeng, Polymer Nanocomposite Foams by Using Supercritical CO ₂,unpublished.

[0106] 16. L. J. Lee, C. Zeng, X. Han, D. L. Tomasko, and K. W.Koelling, Polymer Nanocomposite Foams Prepared by Supercritical FluidFoaming Technology, unpublished.

[0107] 17. X. Han, C. Zeng, L. J. Lee, K. W. Koelling, and D. L.Tomasko, Processing and Cell Structure of Nano-Clay ModifiedMicrocellular Foams, unpublished.

What is claimed is:
 1. A method for forming a polymeric nanocompositefoam, said method comprising the steps of: providing a mixturecomprising: a polymer, an organophilic clay, and a blowing agent; andprocessing said mixture so as to cause formation of cells, therebyforming a polymeric nanocomposite foam.
 2. The method according to claim1 wherein said mixture comprises at least 1% by weight of said blowingagent.
 3. The method according to claim 1 wherein said mixture comprisesat least 4% by weight of said blowing agent.
 4. The method according toclaim 1 wherein said mixture comprises at least 7% by weight of saidblowing agent.
 5. The method according to claim 1 wherein said mixturecomprises at least 0.5% by weight of said organophilic clay.
 6. Themethod according to claim 1 wherein said mixture comprises at least 5%by weight of said organophilic clay.
 7. The method according to claim 1wherein said mixture comprises at least 10% by weight of saidorganophilic clay.
 8. The method according to claim 1 wherein saidmixture comprises at least 20% by weight of said organophilic clay. 9.The method according to claim 1 wherein said polymer is selected fromthe group consisting of polystyrene, poly(methyl methacrylate),polypropylene, nylon, polyurethane, elastomers, and mixtures thereof.10. The method according to claim 1 wherein said organophilic clay isdispersed throughout said polymer such that a x-ray diffraction patternproduced from said mixture is substantially devoid of an intercalationpeak.
 11. The method according to claim 1 wherein said organophilic clayis dispersed throughout said polymer such that a x-ray diffractionpattern produced from said mixture contains an intercalation peak. 12.The method according to claim 1 wherein said organophilic claycomprises: a smectite clay; and a compound having the formula:

wherein: R1 is (CH)_(n) wherein n ranges from 6 to 20; R2 is a chemicalstructure having a terminal reactive double bond; R3 is an alkyl group;and R4 is an alkyl group.
 13. The method according to claim 12 wherein nis 15, R3 is CH₃, R4 is CH₃, and R2 is:


14. The method according to claim 12 wherein said smectite clay isselected from the group consisting of montmorillonite, hectorite,saponite, laponite, florohectorite, and beidellite.
 15. The methodaccording to claim 1 wherein said blowing agent is a supercriticalfluid.
 16. The method according to claim 1 wherein said blowing agent issupercritical carbon dioxide.
 17. The method according to claim 1wherein said polymeric nanocomposite foam has an average cell size lessthan about 20 microns.
 18. The method according to claim 1 wherein saidpolymeric nanocomposite foam has an average cell size greater than about15 microns.
 19. The method according to claim 1 wherein said polymericnanocomposite foam has an average cell density greater than about 1×10⁶cells/cm³.
 20. The method according to claim 1 wherein said polymericnanocomposite foam has an average cell density greater than about 1×10⁹cells/cm³.
 21. The method according to claim 1 wherein said polymericnanocomposite foam is closed cell foam.
 22. The method according toclaim 1 wherein said polymeric nanocomposite foam is open cell foam. 23.A polymeric nanocomposite foam produced in accordance with the method ofclaim
 1. 24. A polymeric nanocomposite foam, said polymericnanocomposite foam comprising: a polymeric portion; an organophilicclay, said organophilic clay dispersed throughout said polymericportion; and a plurality of cells dispersed throughout said polymericportion.
 25. The polymeric nanocomposite foam according to claim 24wherein said polymeric portion comprises a polymer selected from thegroup consisting of polystyrene, poly(methyl methacrylate),polypropylene, nylon, polyurethane, elastomers, and mixtures thereof.26. The polymeric nanocomposite according to claim 24 wherein saidorganophilic clay is dispersed throughout said polymeric portion suchthat an x-ray diffraction pattern produced from said polymericnanocomposite foam is substantially devoid of an intercalation peak. 27.The polymeric nanocomposite according to claim 24 wherein saidorganophilic clay is dispersed throughout said polymeric portion suchthat a x-ray diffraction pattern produced from said polymericnanocomposite foam contains an intercalation peak.
 28. The polymericnanocomposite according to claim 24 wherein said organophilic clayportion comprises: a smectite clay; and a compound having the formula:

wherein: R1 is (CH)_(n) wherein n ranges from 6 to 20; R2 is a chemicalstructure having a terminal reactive double bond; R3 is an alkyl group;and R4 is an alkyl group.
 29. The polymeric nanocomposite according toclaim 28 wherein n is 15, R3 is CH₃, R4 is CH₃, and R2 is:


30. The polymeric nanocomposite according to claim 28 wherein saidsmectite clay is selected from the group consisting of montmorillonite,hectorite, saponite, laponite, florohectorite, and beidellite.
 31. Thepolymeric nanocomposite according to claim 24 wherein said polymericnanocomposite foam has an average cell size less than about 20 microns.32. The polymeric nanocomposite according to claim 24 wherein saidpolymeric nanocomposite foam has an average cell size greater than about15 microns.
 33. The polymeric nanocomposite according to claim 24wherein said polymeric nanocomposite foam has an average cell densitygreater than about 1×10⁶ cells/cm³.
 34. The polymeric nanocompositeaccording to claim 24 wherein said polymeric nanocomposite foam has anaverage cell density greater than about 1×10⁹ cells/cm³.
 35. Thepolymeric nanocomposite according to claim 24 wherein said polymericnanocomposite foam is closed cell foam.
 36. The polymeric nanocompositeaccording to claim 24 wherein said polymeric nanocomposite foam is opencell foam.